In mobile radio systems where the mobile radios are allowed to roam freely within the area of the system and have access to the system through a base station selected by them, the registrations of the mobile radios are utilized to keep a record of the coarse location of the mobile radios so that the system is capable of routing calls to the mobile radios. Registration can also be used to prevent the access of unauthorized users to the system. The mobile radio performs a registration whenever it changes the registration area by sending a registration message to a base station selected in accordance with a predetermined selection procedure. The registration message is then forwarded to the mobile radio exchange. In some mobile radio systems, such as the NMT (Nordic Mobile Telephone), the mobile radio has to wait for a registration acknowledgement from the mobile radio exchange before it is allowed to attempt to register through another base station. If the mobile radio does not receive a registration acknowledgement, it informs the user about the situation. The problem with unsuccessful registration is thus left to the user. In some other mobile radio systems, such as the Mobile Private Trunking networks complying with the specifications MPT 1327 and MPT 1434 of the British Department of Trade and Industry, attempts have been made to improve the registration procedure from the user's viewpoint so that if a registration to one mobile radio exchange is not successful, the mobile radio has a chance to try to register, very rapidly, to another mobile radio exchange. If the mobile radio attempts to register to several mobile radio exchanges within a short period of time, the mobile radio system may not be able to determine the actual order of the registration messages received by the different mobile radio exchanges, and so the location of the mobile radio is ambiguous. In this kind of system, the failure of the registration or the disordering of successive registrations may result from, e.g., the radio path delays and interferences, the delays and interferences of the internal transmission paths of the central network, and the registration frequency of the mobile radios.